1. Field of the Invention
The invention relates to a cation-selective sensor which has a cation-selective coating. As a result of cations to be detected coming into contact with this layer, a detectable change in the electrical properties of the layer is brought about.
2. Description of the Prior Art
For the determination of ions in solutions, use is frequently made of the potentiometric ion-selective electrode (Cammann, K., Die Arbeit mit Ionenselektiven Elektroden [Working with ion-selective electrodes], 2nd ed., Springer Verlag: Berlin, Heidelberg, New York, 1977). Ion-selective electrodes are electrochemical sensors with which the concentration or activity of specific ions can be determined by means of a potential difference. The ion-selective potential difference occurs at the phase boundary between active electrode material/electrolyte and depends according to the Nernst equation on the activity of a specific ion in the solution. One example of sensors of this type are ion-selective field-effect transistors (for example DE 29344005 C2).
Unlike the case of resistance and capacitance, the absolute values of the electrical potential have no physical meaning, since the potential can only be defined in relation to a reference value. In electrochemistry, such a reference value is customarily given by the potential of the reference electrode. The need for a reference electrode is the critical disadvantage with the use of potentiometric measurements for the determination of ion activities in solution.
Another fundamental limitation with the potentiometric analysis methods relates to the composition of the ion-selective membrane. The requirements made of the nature of the specific binding and/or of the complexing sites within the membrane should be such that the potential difference at the membrane/solution interface is generated selectively as a function of the presence of a particular species in the solution. For example, this binding should not be too strong, in order to permit sufficiently fast exchange of the detected species between the membrane phase and the solution.
Besides the potentiometric analysis methods, the most frequently used electrochemical analysis methods are those which measure the current through a suitably prepared or modified conducting or semiconducting working electrode. The potential of this electrode is set by that of the reference electrode. The measured current results from the electrochemical redox reaction which takes place at the working electrode/solution interface. In addition to the reference electrode which is needed, the use of this measurement method is further limited by the fact that the measured species must be electroactive at the working potential applied to the working electrode. Furthermore, this potential must be different from that of the interfering species. The latter point often causes a problem since many chemical compounds or large groups of chemical compounds have very similar redox properties. In addition, the required electrode potentials for many compounds lie outside the range which is practically usable.
The non-electrochemical methods usually employed for the specific recognition of charged and neutral species include the various types of liquid chromatography. In this case, the sample to be analysed is brought into contact with a so-called stationary phase, for example a polymer layer, which specifically binds or retains the detected species. The strength of this binding determines the retention time of the analyte within the chromatography column. When tailor-made stationary phases are used, very many species can be identified. However, this type of analytical measurement arrangement is highly complex and very expensive.
A further possibility for the determination of ions in solutions is given by ion-selective optodes. Ion-selective compounds and indicators, which contain structural elements that change their optical properties in the UV/VIS range, can be used as chromoionophores and fluoroionophores in corresponding ion-selective sensors with optical signal transmission. An overview of the way in which ion-selective optodes function is given in the following articles: K. Seiler, Ionenselektive Optodenmembranen [Ion-selective optode membranes], Fluka Chemie AG, Buchs, Switzerland (1991), ISBN 3-905617-05-6, W. Morf, K. Seiler, P. P. Sorensen, W. Simon, Ion-Selective-Electrodes, Vol. 5, Pergammon Press, Oxford, New York, Akademiai Budapest (1989), p. 141.
Interaction of ions with the chromophore components, or fluorescent components, in a membrane which is applied to an optical transducer system, leads to absorption or excitation of the fluorescence, and it has in this way been possible to develop chemical sensors for colourless or non-fluorescent substances.
The optical sensor systems have fundamental disadvantages. For example, the optical systems are disturbed by background light and, compared with electrochemical sensors, have relatively narrow dynamic measurement ranges. Furthermore, the long-term stability of immobilized components is limited by photolytic breakdown and leaching, and the response times of optical sensors are relatively long. Further disadvantages of ion-selective optodes include incompatibility with microelectronics and the lack of possibilities for integration.
The synthesis of chromophore compounds or fluorescent compounds is very time-consuming and cost-intensive, which is likewise highly disadvantageous.
A further possibility for the determination of ions in solutions is given by test rods and test papers. This involves a microchemical investigation method, in which chemical reactions, visible to the naked eye, of small quantities of elements (in the form of their ions) or compounds can be identified. The analytes can be identified by virtue of their colour reactions (colour changes). In this case, all the reagents needed for the specific detection reaction are applied to a support and, on exposure to an aqueous analyte solution, the analyte in question can be assigned to a concentration range according to the intensity or the hue of the respective coloration. The reagents used in test papers and test rods for the specific colour reactions or colour changes are also used in colorimetric test systems. In colorimetry, the colour intensity of a sample solution is visually compared with the intensity of standard solutions whose concentrations are known. Problems and disadvantages are found with the accuracy in these determination methods, which often only give semiquantitative conclusions. It is not possible to use test strips in on-line measurement systems, and so test strips cannot function as sensors. EP 0 153641 A2 presents the structure and measurement method for some test strips.
Another important class of analytical methods for the detection of charged or uncharged species in a gas or liquid medium employs the measurement of resistance or capacitance. Variations in the conductance or the dielectric properties of a layer of a sensitive material are exhibited as a function of the interactions with the detected species. In the field of gas detection, resistive and capacitive sensors are thus widely used.
In contrast to this, the use of such sensors is encountered only infrequently for chemical analyses in liquids. Measurements of the total conductance of electrolyte solutions are of only limited analytical meaning, because they generally lack specificity. Notwithstanding, in GB 2204408 A, R. S. Sethi et al. described a conductimetric enzyme biosensor which has finger-like interdigital electrodes (IDEs) that are covered by a membrane of immobilized urease. When urea is present in the test solution, the use of densely arranged electrodes makes it possible to measure the conductance of the solution with which the enzyme layer is saturated, so long as the conductance changes specifically with respect to the urea hydrolysis which is catalysed by urease. The shortcomings of biosensors of this type includes the drastic reduction in the sensitivity of the biosensor as the buffering capacity and/or ion strength (conductance) of the solution increases.
WO 93/06237 describes the use of IDEs for measuring the change in conductance of a layer of electroactively conducting polymer (polyaniline, polypyrrole). These changes result from the interaction of the functional redox groups of the polymer with the species of interest present in the solution, or with species which result from an enzymatic reaction in the layer of immobilized enzyme which is applied to the top of the layer of the said polymer.
U.S. Pat. No. 4,334,880 describes an analyte-specific resistance meter, an electrically conducting or semiconducting layer of polyacetylene being used as an analyte-specific layer.
In GB 21 37 361, L. S. Raymond et al. describe a capacitive detection arrangement which contains the following components:
1. a capacitor consisting of two IDEs; PA1 2. a first layer of electrically insulating material, which covers the electrically conducting electrode and shields it from the solution to be analysed; PA1 3. a second layer of a material, which covers the first layer, the second layer being permeable to a specific non-aqueous substance in a solution, which, through its entry into the electric field between the IDEs, causes a change in the capacitance of the capacitor. PA1 1. No reference electrode is needed in the sensors according to the invention, since the measurements of electrical properties such as conductance or admittance are, in contrast to e.g. potential measurements in potentiometry, absolute measurements. PA1 2. The novel functional mechanism of the sensor avoids the interfering effect of ions in the sample solution on the response of the sensor. PA1 3. The use of ion exchange as a novel functional mechanism for the sensor improves the response of the sensor (reversibility, selectivity, stability). PA1 4. A wide range of new materials can be used for sensor optimization and sensor production. PA1 5. The novel functional mechanism of the sensor greatly suppresses the effect of the sample matrix on the measurement results, and substantially facilitates the sample preparation. PA1 6. In contrast to potentiometric sensors, these sensors allow the determination of analyte concentrations in solutions with very high ion strength. PA1 7. With the sensors, it is possible to measure the solution composition in closed vessels, for example in sealed glass ampoules, using contactless measuring techniques. PA1 8. The sensors can be designed as complete solid-state systems with a high level of integration. This allows extensive miniaturization and ensures compatibility with microelectronics. PA1 noble metals (Ag, Au, Pt, Pd, . . . ); PA1 other metals with sufficient chemical stability (Ni, Ta, Ti, Cr, Cu, V, Al, . . . ); PA1 conductive pastes and epoxy resins containing metal particles or graphite particles; PA1 carbon-based materials (carbon fibres, glassy carbon, graphite); PA1 heavily doped silicon (poly-Si); PA1 conductive polymers (polypyrrole, polyaniline, polyacetylene, . . . ); PA1 conducting polymers which contain metal particles or graphite particles. PA1 (a) increase/decrease in the cross-linking of the matrix polymer, or PA1 (b) configurational molecular change in the components of the membrane layer. PA1 20-80% by weight of polymer material, preferably 30-35% by weight PA1 20-80% by weight of plasticizer, preferably 60-65% by weight PA1 1-10% by weight of cation-selective coupling element, preferably 1-5% by weight PA1 1-10% by weight of component with acid/base properties, preferably 1-5% by weight
The second layer contains, for example, valinomycin which is selectively permeable to potassium ions. The interdigital electrodes measure the changes in the capacitance as a result of the specific uptake of ions into the valinomycin layer.
GB 21 37 361 does not give any description of the membrane composition, that is to say there is a lack of indication as to the conditions needed to ensure the required permeability of the sensitive second layer in relation to the species of interest. In addition, conditions of this type greatly limit the number of species that can be detected. The need to shield the conducting electrodes with an insulating layer makes it more difficult to produce the transducer on account of the stringent requirements for the quality of a layer of this type, and at the same time worsens the sensitivity of the sensor. A further problem is that it is not possible to rule out an abrupt change in the dielectric constants of the measuring layer as a function of the composition of the solution to be analysed.